1. Field of the Invention
The present invention relates to a battery, in particular a traction battery.
2. Description of the Prior Art
From today's perspective, it is likely that hybrid and electric vehicles will be coming into increasing use in the future. In these vehicle concepts, quite different demands are made of the batteries, compared with the demands made of batteries in today's 12-Volt on-board electrical system. In the present specification, batteries for use in hybrid and electric vehicles are called traction batteries, since they are used for supplying electrical drives. The basic circuit diagram of one such battery is shown in FIG. 2. To achieve the power and energy data demanded for hybrid and electric vehicles, individual battery cells are connected in series and in part in parallel as well. A traction battery based on lithium-ion battery cells for use in hybrid vehicles could be constructed for instance as follows:
Overall Traction Battery:                Rated voltage: 360 V        Capacity: 4 Ah        Energy: 1440 Wh        
Individual cell (lithium-ion):                Rated voltage: 3.60 V        Capacity: 4 Ah        Energy: 14.4 Wh        
The battery comprises a series circuit of 100 cells.
The basic circuit diagram of a drive system for hybrid and electric vehicles is shown in FIG. 3. The electric motor 17 is supplied via an inverter 16. Via the inverter 16, it is possible—for instance in braking operations—for energy to be fed back into the battery 15 as well. The inverter 16 is typically embodied with a smoothing capacitor on its direct voltage side, in order to buffer the input voltage.
Besides the battery cells, the traction battery shown in FIG. 2 has the following further function groups as well:                charging and disconnecting device 3 at the positive pole 4 of the battery        disconnecting device 7 at the negative pole 8 of the battery        surface plug 9        
These function groups have the following tasks:
With the two disconnection switches in the disconnecting devices at the positive pole 4 and at the negative pole 8 of the traction battery, the battery cells can be switched off at both poles. This is also called two-pole shutoff of the battery cells. Thus when the vehicle is stopped, or in safety-critical situations (such as an accident), the battery can be disconnected from the traction on-board electrical system of the vehicle, and in the driving mode it can be connected into the traction on-board electrical system.
The charger at the positive pole of the battery has the task of limiting the compensation currents in the traction on-board electrical system upon connecting-in of the traction battery. In such a connecting-in operation, first the disconnection switch in the disconnecting device 7 is closed at the negative pole 8 of the battery, and the charging switch in the charging and disconnecting device 3 is closed at the positive pole 4 of the battery The disconnection switch in the charging and disconnecting device 3 at the positive pole 4 of the battery is first opened. The smoothing capacitor of the inverter 16 is thus charged via the charging resistance of the charging and disconnecting device 3 at the positive pole 4 of the battery. If the voltage at the smoothing capacitor in the inverter 16 has nearly the total voltage of the series-connected battery cells, the disconnection switch in the charging and disconnecting device 3 at the positive pole 4 of the battery is closed. With this procedure, the compensation currents in the traction on-board electrical system can be limited such that both the battery cells and the smoothing capacitor in the inverter 16 are not operated with impermissibly high currents.
For safety reasons, during maintenance work on a battery the surface plug 9 has to be unplugged. This can be ensured for instance by providing that the battery housing can be opened only if the surface plug 9 has first been unplugged. As a result, the battery cells are safely disconnected in single-pole fashion even if because of a malfunction the two disconnection switches described above have not opened.
Markedly more stringent demands in terms of reliability are made of traction batteries than of starter batteries that are usual today. The background of this is that in an electric vehicle, for instance, a failure of the traction battery leads to a so-called “dropout”. The high availability required is hard to achieve a traction battery shown in FIG. 2. The reason for this is that the failure of a single battery cell leads to the failure of the entire traction battery. The failure rate of a battery with a series circuit of individual cells can be ascertained as follows:failure ratetraction battery=1−(1−failure ratecell)number of cells  (1)
For a traction battery with 100 cells, which in the period of time in consideration has a failure rate of 100 ppm/cell, is:failure ratetraction battery=1−(1-100 ppm)100=9.95%  (2)
At very low failure rates of the battery cells within the period of time in consideration (for instance, failure ratecell <1%), the failure rate can be calculated approximately as follows (breakdown of the exponential series development of the binomial series after the first term):failure ratetraction battery≈number of cells*failure ratecell  (3)
Thus the failure rate of the traction battery in question is nearly 100 times as high as the failure rate of an individual cell. Given the values needed for the failure rate of the total battery, the failure rate of the individual cells accordingly has to be lower by a factor of approximately 100. If for a total battery with 100 series-connected cells for a certain period of time a failure of 100 ppm is demanded, the cells in that period of time must have a failure rate of 1 ppm. This is a demand that is extremely difficult to meet.
The object of the invention is to increase the reliability of traction batteries in hybrid and electric vehicles. The traction battery should also still be available with limited power to the traction on-board electrical system.